Superconductivity, the property whereby certain materials have the ability to lose electrical resistance at extremely low temperatures, was discovered in 1911. However, significant commercial exploitation of superconductivity has only recently occurred.
With the development of niobium-titanium materials which can be drawn into wires, which have a superconducting transition temperature (T.sub.c) above 10K, and which can remain superconducting even in high magnetic fields, superconducting electromagnets have become commercially practical. Despite the great expense inherent in cooling niobium-titanium materials with liquid helium, superconducting electromagnets are now in widespread use in medical devices, scientific research apparatus, and in advanced electronic circuitry.
Many other applications for superconductors have been developed which are not yet commercially feasible because of the cost of cooling superconductor materials presently available in wire and thin film form. Among the prospective $ commercial applications are superconducting electric motors, magnetically levitated trains or other vehicles, long distance low loss transmission of electric power, magnetic energy storage, and much smaller and more powerful computers. Superconductors capable of remaining in their superconductive state above the temperature of liquid nitrogen, 77K, would make many of these applications practical.
In 1986 Mueller and Bednorz discovered a new class of superconducting materials having a much higher T.sub.c than niobium-based materials. Intensive, world-wide research since then has produced related materials having a T.sub.c as high as 125K. Among these materials are superconductors which have a perovskite crystal structure and the basic formula R.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, where R is yttrium or a rare earth metal and x is between 1.0 and 0. These compounds are colloquially known as "1-2-3" compounds because of the ratio of R to barium to copper.
Early 1-2-3 superconductors were commonly prepared as powders by mixing powdered yttrium or rare earth oxide, barium carbonate and copper oxide in appropriate proportions. The mixture was finely ground followed by calcining in air or oxygen at atmospheric pressure and at a temperature as high as 950.degree. C. for approximately 4 to 24 hours. The calcined material was then typically reground and recalcined at least one additional time.
The product resulting from the above-described process generally contained some undesirable impurity phases in addition to the 1-2-3 phase. When the product was subsequently sintered at temperatures between about 850.degree. C. and 1100.degree. C., the impurity phases tended to react with each other or with the 1-2-3 phase to form liquids. Liquids phase relations of this nature are well known and described in Aselage, et. al., "Liquids Relations in Y-Ba-Cu Oxides:, J. Mater. Res. 3, 1279 (1988); and Ullman, et. al., "Effect of Atmosphere and Rare Earth on Liquidus Relations in RE-Ba-Cu Oxides", J. Mater. Res., 4, 752 (1989).
The presence of liquids along with the 1-2-3 phase in the product can have serious deleterious effects, as enumerated below, on the properties and performance of the 1-2-3 superconductor, and particularly on the critical current density (J.sub.c) of the 1-2-3 superconductor. Frozen liquids can coat the grain boundaries of the 1-2-3 phase, thereby forming an electrically resistive non-superconducting barrier on the 1-2-3 phase. The liquids can cause significant grain growth of the 1-2-3 phase during sintering. This in turn can result in undesirable microcracking of the grains due to the large anisotropy in the thermal expansion of the 1-2-3 phase. If the 1-2-3 superconductor is to be coated on a substrate, the liquids can react with the substrate, thereby corroding and embrittling the substrate. The liquids can also transport substrate components into the superconductor coating, which can dope the superconductor and significantly reduce the values of T.sub.c and J.sub.c.
There presently exists considerable interest in coating metallic substrates, such as Ni or Ni-alloys including Duranickel 301, with 1-2-3 superconductor powder and sintering the powder thereon. Accordingly, it has been found desirable in the manufacture of coated substrates to use superconductor powder that is substantially free of any impurity phases capable of forming detrimental liquid phases at the elevated sintering temperatures. Furthermore, it has been found in the continuous production of superconductor coated metal fiber, such as described in co-pending U.S. Pat. application, Ser. No. 586,450, that it is desirable to sinter the superconductor powder coating the fiber in a relatively short time frame, i.e., approximately 0.1 to 10 minutes.
In order to sinter a relatively pure 1-2-3 powder in a short time, the sintering temperature should be as high as possible without substantially melting any of the powder. Thus, the sintering temperature should be near, yet below, the melting onset temperature of the powder. Given these constraints, it is apparent that the melting transition width of the superconductor powder should desirably be as small as possible so that the sintering temperature can approach the melting transition midpoint without melting the powder.
As such, there is a need for superconductor powders which have a high T.sub.c and which are capable of being sintered at high temperatures without melting. It is therefore an object of the present invention to provide a process for manufacturing a superconductor powder having a high T.sub.c. It is another object of the present invention to provide a process for manufacturing a single phase superconductor powder containing substantially no impurity phases which could lead to undesirable liquid phases at elevated sintering temperatures. Another object of the present invention is to provide a process for manufacturing a superconductor powder having a relatively small melting transition width. Further, it is an object of the present invention to provide a process for manufacturing large quantities of a superconductor powder having a fine particle size which is relatively easy and cost-effective to use.